Various abbreviations that may appear in the specification and/or in the drawing figures are defined as follows:
3GPP third generation partnership project
EDGE enhanced data rates for global evolution
GSM global system for mobile communications
GERAN GSM/EDGE radio access network
GPRS general packet radio service
(E)GPRS enhanced GPRS
UTRAN universal terrestrial radio access network
UE user equipment
MS mobile station
EUTRAN evolved UTRAN (also known as LTE)
LTE long term evolution
OFDMA orthogonal frequency division multiple access
SC-FDMA single carrier, frequency division multiple access
UL uplink
DL downlink
RACH random access channel
LSE least square error
BLER block error rate
SNR signal to noise ratio
BPSK binary phase shift keying
QPSK quadrature phase shift keying
QAM quadrature amplitude modulation
VoIP voice over internet protocol
In cellular communication systems, regardless of the multiple access method or modulation method that is used, there is a need for a timing advance loop to exist between a network access node (e.g., a base station or a Node B or, in LTE, an eNB) and a mobile device (e.g., a MS or UE). For example, a timing advance loop is defined for GSM/EDGE systems, OFDMA systems, as well as for SC-FDMA systems, such as the LTE system. The purpose of the timing advance loop is to enable the alignment of UL signals (transmissions from a mobile device) at the network access node within some time resolution. By time aligning the UL signals the network access node receiver design can be relaxed somewhat, and in some multiple access schemes, such as in OFDMA, signal orthogonality between UL transmissions from a plurality of mobile devices can be maintained with greater ease.
A general procedure for initial synchronization may be summarized as follows:
1. The mobile device synchronizes to a base station DL transmission to achieve a rough estimate of timing synchronization. The effect of signal propagation delay is not included in this estimate.
2. The mobile device sends a message (e.g., a RACH message) which the base station detects and estimates timing from.
3. The base station signals the mobile device to adjust its timing so that subsequent transmissions are aligned at the base station receiver.
4. The mobile device adjusts the transmission timing accordingly.
After the initial synchronization procedure is carried out the timing of the mobile device transmissions may change due to changes in propagation delay. The same result is achieved by movement of the mobile device as by the presence of a frequency error between oscillators of the mobile device and the base station. The timing advance loop adjusts for these changes in propagation delay. The procedure is as follows:
1. The base station estimates the changes in delay by the means of a delay estimation method.
2. The base station signals to the mobile device to adjust the transmission timing of the mobile device to compensate for the estimate change in propagation delay.
3. The mobile device makes the requested adjustment to the transmission timing.
In conventional approaches the base station measures the channel impulse response from a longer time window than would have been required without delay estimation. This has been done to in order to obtain information about the impulse response both before and after the expected time-of-arrival of the signal transmitted by the mobile device. The impulse response energy from before, on-time and after the expected time-of-arrival have also been averaged for a period of bursts. The decision to then signal the mobile device to either delay or advance its timing has been performed based on a maximum impulse response energy.
The conventional time alignment techniques are not optimum, and improvements are needed.